1. Field of the Invention
This invention relates generally to improvements in methods and apparatus for producing hot mix asphalt and more particularly, but not by way of limitation, to methods and apparatus wherein hot mix asphalt is produced from virgin aggregate and a previously manufactured and laid asphalt mix which has been reduced to form recyclable aggregate.
2. Description of the Prior Art
For many years aggregate dryers used in batch type asphalt plants, as well as combination dryers and mixers used in commonly called drum mix asphalt plants, have used flights around the interior of the drum to cascade aggregate through a flame or hot gases for heating, and in drum mix plants to further mix the heated aggregate with asphaltic oil. A typical drum would be tilted downwardly toward the discharge end such that the material moving through the drum may, for example, be maintained in the drum three minutes and the drum may, for example, be rotated about its longitudinal axis at a speed of eight R.P.M. Theoretically, each particle of aggregate would then be lifted by the flighting and cascaded twenty four times while traversing the length of the drum. Historically, except at the extreme ends of the drum, the flighting was designed in the same manner throughout the length of the drum. Thus, if the hot section or zone of the drum (where the flame was present) was about 7/8 of the length of the drum, the aggregate would be cascaded through the flame only eight times and would be cascaded through hot gases in the remainder of the drum sixteen times. The result has been that what are now considered excessive BTU's are required to adequately dry and heat the aggregate. In a typical drum mixer, the fuel required to dry and heat each ton of aggregate having a moisture content of 5% is 1.65 gallon and the air flow is 125 CFM per ton.
It is also known to produce hot mix asphalt from virgin aggregate and from a recyclable aggregate which has been reclaimed from a previously manufactured and laid hot mix asphalt. As has been described in the above referenced patent application, the new hot mix asphalt can be produced in an apparatus having a rotating mixer drum in which virgin aggregate and recyclable aggregate are mixed with asphaltic oil to produce new hot mix asphalt. In the apparatus described in the aforementioned U.S. patent application, virgin aggregate is introduced into an input end of the drum and dried by a flame which is similarly introduced into the input end of the drum. The recyclable aggregate, which contains asphaltic oil, is introduced at a medial portion of the drum beyond the flame so that decomposition of the asphaltic oil contained in the recyclable aggregate is minimized. Thus, the asphaltic oil used in carrying out such projects as the paving of a roadway or the like is reduced.
In the above mentioned patent application, an improvement in the transfer of heat to the virgin aggregate is obtained by arranging the flighting in the hotter zone of the drum such that the time of exposure of the aggregate to the flame, is increased. That technique is a substantial improvement in heating efficiency and is the forerunner of the present invention.
Another problem also occurs in the operation of an apparatus in which hot mix asphalt is produced in a rotating drum. Virgin aggregate contains small particles, or fines, some of which are not incorporated into the bulk of the hot mix asphalt produced in the drum. The fines which are not incorporated in the mix are carried out of the drum by the secondary air flow and are captured by means of a filtration system for that purpose. It will be clear that to the extent that fines can be incorporated into hot mix asphalt produced in the drum, energy may be saved by reducing the size of such an exhaust fan and materials can be conserved by reducing the quantity of virgin aggregate which has previously been wasted in the preparation of new asphalt.